The easiest way to do this is to put a jumper wire from the hi beam plug to the low beam plug, with a diode in between.

Result:
low beams on = low beams on
hi-beams on - hi and low beams on

I would advise on using relays and larger wiring to get you the brightest lights possible:





I ran this set-up on my Bronco for the auxillary lighting for at least 10 years with no issues whatsoever.